In recent years, a new type of multifocal ophthalmic lens has been introduced into the marketplace. This lens is generally known as a progressively variable focal power lens, or, more simply, as a progressive power lens, and it is designed to provide multifocal lens characteristics without any sharp dividing line or other discontinuity between the various portions of the lens. Specifically, such lenses are characterized by having a progressive corridor portion positioned between the distance and reading portions of the lens and having a continuously varying focal power beginning with a focal power equal to that of the distance portion where it merges with and into the distance portion and ending with a focal power equal to that of the reading portion where it merges into the reading portion of the lens. By gradually varying in focal power in this way, the sharp dividing lines normally found between the different portions of the multifocal lens are eliminated, and a lens is provided that is continuous throughout, both physically and optically.
Typical of the progressively variable focal power lenses found in the prior art are those described in U.S. Pat. Nos. 3,687,528; 3,711,191; and 4,056,311.
Because progressive power lenses have no readily visible multifocal features inherent in their design, such as the boundary lines found in conventional multifocal lenses, there is nothing on the lens that can be used, by a surfacing laboratory or by an optical dispenser, for example, as a reference to locate various surface features.
Partially in recognition of this problem, it has become the practice of progressive lens manufactures to place two very faint, generally circular marks on the lens surface which can be used in conjunction with an appropriate fixture to locate the optical centers of the distance and reading portions of the lens. Although this is suitable for use by the optical dispenser to measure and test a finished lens, and for other purposes, it is not very suitable for use by the surfacing laboratory which needs appropriate markings as an aid in surfacing layout and finishing layout procedures.
It is also taught in the prior art to place short lines or hash marks at appropriate locations on the lens surface to be used, in conjunction with an appropriate fixture, to locate and ink the distance portion optical center and the 0.degree.-180.degree. lines (which are used to indicate the extent to which the lens is to be rotated to accommodate the left or right eye). One disadvantage of this technique is that an appropriate fixture is needed which must be visually lined up with respect to the hash marks to permit the lens to be marked and care must be taken to ensure that the marks are placed accurately. Also, in the case of plastic lenses (most progressive power lenses are manufactured of plastic) wherein the marks are replicated on the lens surface from a mold surface against which the lens is cast, substantial care is also necessary to ensure that the mold surface is accurately positioned in the mold assembly so that the marks will be accurately positioned on the lens surface. Also, because the hash marks are on the lens surface, they decrease the effective useable diameter of the lens.